Method of producing magnesium



1945- T. H. MccoNlcA, 30

METHOD OF PRODUCING MAGNESIUM Filed Sept. 20, 194-4 2 Sheets-Sheet l INVEN TOR.

Th 00705 H M- Con/ca ATTORNfYS Dec. 25, 1945. T. H. M CONICA, 302,391,727

METHOD OF PRODUCING MAGNESIUM Filed Sept. 20, 1944 2 Sheets-Sheet 2 N ws, N INVENTOR. LL Thomas H McCofi/ta M MM A fro /vars PatentellDee.25,1945

UNITED I STATES rArsNrmrrlcE so e I ammonor raonncnve monxsrum Thomas a.llcconica, m,

I sncrto mulch,-

'l.'he Dow Chemical Company, Mld.

chigan land, Mich a corporation of ill Application September 20, 19,Serial No. 554,975

claims fflhisinvensionisconoernedwiththeth sl production of metallicmagnesium.

The interaction of solid magnesia and carbon to form a vapor mixture or,magnesiumand car- 'bonmonoxideisatrueequilibriumprccess in which, atoperating temperatures, the forward and reverse reactions both proceedat appreciable rates. In consequence. it is prerequisite to the successof any carbothermic magnesium process that theoperating conditions bechosen so-that the rate of magnesium evolution fromthe charge,

- i. e. of the forward reaction, is greatin comparison tothat of thereverse reaction. At least two diiferent ranges of ture and pressurehave been recommended in the art.

Inoneprocesaachargemixtureofmagnesia and carbon is heated at atmosphericpressure to a temperature well above 1900' c. 'I'he magneslum-carbonmonoxide vapor mixture thus formed is led to a condenser at atmosphericpressure and is quenched rapidly to a temperature below 850 C. toproduce solid magnesium. In this method. a high rate of magnesiumevolution is achieved, but great diiiiculty is experienced in conveyingthe vapor mixture to the condenser and quenching it without undue lossof magneslum by reversion to magnesia. Thus, at the opthis materialtends to deposit in the condenser inlets, causing mechanical plugging.In addition, within the condenser itself, in which the vapor mixture isshock-chilled through the reversion 7 temperature range of 1750 to 6500., dimculty is experienced in effecting the quenching with suilcientrapidity to avoid considerable further v formation of magnesia andcarbon. As a result of these two losses magnesium productioneiliciencies exceeding 70 to 80 per cent are rarely realised.

In another known process, the charge mixture is heated under vacuum,usually at 0.001 atmosphere or less, at a temperature above 1400 C., andthe evolved vapors are passed to a coolersoneatthesamepressureandtherechilledto in: a hearth i of .broken coke.

icl. 75-67) p magnesium by reversion isnot excelsivabut th rate ofreaction must be kept undesirably low in order to avoid transport of;charge particles to the condenser by entrainment'in the vapor mix- 5ture. Moreover. serious practical diiilculties are encountered inoperating alarge furnace at high vacuum. For these reasons. the processhas not been satisfactory in large-scale production.

It is accordingly an object oiwthe present in- 1o vention to provideanimproved carbothermal process for making magnesium in which the metalis produced at a high rate with good eflciency and in which many of thetroubles of prior practies are avoided.

In the process of the invention, the magnesiacarbon charge mixture isfirst heated'at a pressure of at 1east-0.5 atmosphere to a temperaturesufficient to form a vapor mixture of carbon monoxide and magnesium.This mixture is-then etgo panded to a reduced pressure between 0.1 andabout 2.5 inches of mercury absolute, and is quenched at this latterpressure to a temperature below about 650 C. By operating in thismanner. formation of magnesium-carbon monoxideggvapormixturemaybecarriedoutinafurnace of simple construction, at highrate and with minimum entrainment of charge particles. At

the same time. the expansion of the vapors lowers the upper reversiontemperature and reduces 80 the rate of reversion so substantially thatthey may readily be transported to the condenser and even cooledintentionally without appreciable Ior- 'mation of magnesia and carbon orplu ng of the vapor conduit. In addition, the quenching operation, beingat drastically reduced pressure,

takes place with little if any reversion of the magnesium. Exceedinglyhigh overall-recovery ,eiiiciencies may be realized.

' The invention may be further explained with 40 reference to theaccompanying drawings, in

which Figure 1 illustrates, in vertical partial crosssection,onearrangement of apparatus for ,carrying out the expansion andquenching steps of the new process; and

l 'igure2isasideviewofthefurnaceotl'igure 1, showing auxiliarygas-circulating equipment.

In the equipment illustrated, the magnesiumcarbon monoxide vapor mixtureis generated in an arc furnace 3 formed of a gas-tight steel shell 4.lined with refractory carbon blocks 0 and hav- The furnace is heatedelectrically by arcs struck between the condense magnesium. 'Inthismethod, loss of hearth and graphite electrodes 'l-which enter throughwater-cooled gas-tight glands 8. Charge mixture is fed'in through anupper inlet provided with a variable-speed rotary lock 10.

The vapor mixture leaves the furnace through a narrow-throated expansionorifice ll formed ina small block of highly refractory'material, such asboron carbide. The diameter of the orifice is ordinarily quite small,being usually only a few inches, evenfor furnaces of very large size.The orifice body Ii is held by a carbon bushing 12 in a socket in acarbon thrust-block I 3 secured in the furnace wall and seated on asteel ring if welded to the furnace shell.

The expanding vapors issuing from the orifice ii enter a conduit I!which leads into a gastight thermally insulated quench chamber I6. Inthis chamber, the vapor stream impinges on successive falling streams I!of a quench liquid,

in which the magnesium is shock-chilled and condensed. Thenon-condensable carbon monoxide is continuously exhausted through asuction stack l8.

Any solid deposit forming in the vapor orifice ll may be poked loose byan alloy steel reamer rod I. of diameter slightly less than that of theorifice. This rod is mounted slidably opposite the orifice in a glandwelded through a cover flange 2! on the end of the quench chamber.

The quench liquid, preferably a molten leadmagnesium alloy, ismaintained under inert gas protection in a closed insulated reservoir22. A portion of the quench liquid is continuously forced by a pump 23driven by motor 23a through an'insulated line 2l into a distributing box25 formed in the top of the chamber i6 above the quench zone. Thisliquid falls through transverse slots 26 cut in the bottom of the box 25in wide streams I! having the effect of liquid sheets or curtains, andis collected in the bottom of the quench chamber and returned to thereservoir 22 through an insulated drain pipe 21. Part of the quenchliquid containing condensed magnesium is withdrawn continually from thereservoir by a pump 28 driven by motor 280 and is circulated through apipe 29 to a magnesium recovery system not shown, from which themagnesium-depleted liquid is returned for re-use by a pipe 30.

The suction in the stack I8 is created by a vacuum pump 3| and isregulated by a damper 32, which may be rotated by a position controller33 in response to variations in quench pressure conveyed to thecontroller by a gauge line 34 connected into the stack. The carbonmonoxide flowing through the stack 18 enters a cyclone separator 35 toremove any suspended dust before reaching the vacuum pump through a line36.

The carbon monoxide exhausted by the vacuum pump may be vented through avalved line 31, or it may be circulated to storage through a secondvalved line 38 leading into a gas holder 39. Fromthis gas holder, thecarbon monoxide may be returned to the arc furnace through a connectingpipe 40 in which flow is regulated by a valve 4|, this latter beingadjusted by a controller 42 in response to variations in furnacepressure transmitted by a gauge line 43.

In operation of the apparatus illustrated, the arc furnace is maintainedat a temperature of at least 1900 0., preferably 2000 C. or more, and acharge mixture of magnesia and carbon is admitted continually throughthe inlet 9, falling into the hearth 6 and rapidly evolving a vapormixture of magnesium and carbon monox- The quench chamber is maintainedat a pres-- sure in the range 0.1 to about 2.5 inches of mercuryabsolute, preferably about 0.2 to about 1.0 inch, by the action of thepump 3|, the pressure being "automatically regulated by damper 32 tohold the-set pressure. The temperature of the quench liquid fallingthrough the quench chamber is controlled below 650 C. by heat-exchangers(not illustrated) in the reservoir 22. Likewise, .the concentration ofmagnesium in the quench liquid is held roughly constant by regulatingthe recovery system fed by the pump 20. For example, when the quenchliquid is a leadmagnesium alloy, the temperature is preferablycontrolled in the range 500 to 600 C. and the concentration of magnesiumbetween 8 and 15 per cent by weight, the recovery of magnesium beingconveniently carried out by boiling magnesium out of the alloy atreduced pressuqe, in accordance with known practice.

Under the conditions just described, the magnesium-carbon monoxide vapormixture is evolved rapidly from the charge, and fiows toward the orificeII at a relatively low rate, entraining few if any charge particles, andremaining at a temperature at least slightly above its upper reversiontemperature until it enters the orifice. There, because of the largepressure drop, it expands with extreme velocity, entering the pipe I5and at once assumes the pressure in the quench chamber. In this pipe,the temperature of the vapor mixture, which is still essen tially thatof the furnace, is far above the upper reversion temperaturecorresponding to the prevailinglow pressure. The mixture is thusmaintained under substantially non-reversionary conditions at all timesuntil it meets the streams I! of quench liquid. Moreover, even when thevapors are cooled into the reversion range during condensation by thequench medium, the rate of reversion is very low because of the reducedpressure. In addition, excellent intimacy of contact is obtained betweenthe quench liquid and the vapor mixture, since the latter impinges onthe flowing liquid at high velocity. As a result of these factors, themagnesium vapor is, in contrast to prior processes, quenched andcondensed rapidly and effectively, without appreciable reversion tomagnesia and carbon.

By virtue of the fact that the vapor mixture flowing in the pipe I5 isat a temperature far above its reversion range, it may, if desired, becooled substantially before it reaches the quench curtains l1, thusappreciably reducing the cooling load on the quench medium. Thiscooling'is preferably accomplished by radiation, i. e. simply by notinsulating the pipe 15 completely, although posltivecooling, as byinjection of cold carbon monoxide, may be used.

As already stated, in the process-of the invention, the pressure in thefurnace may be controlled at any value above about 0.5 atmosphere.However, it is advantageous to operate with the furnace at, or evenabove, atmospheric pressure,

since at these pressures operation of the furnace used. In addition, atpressures approaching at mospheric, the hazard of possible air leakageinto the furnace is minimized.

In general," it is better to maintain furnace pressure by adding furnacecharge at a high rate rathe than by recirculating large volumes ofcarbon monoxide through the valve 4 I. At the higher charging rates, thepartial pressures of the evolving magnesium and carbon monoxideapproximate the desired total furnace pressure, and the valve 4| is thencalled upon to admit recycled carbon monoxide only to make up forinfrequent diminution in pressure below the set minimum. Under theseconditions, optimum conditions for evolution of magnesium and carbonmonoxide are realized and heat losses due to recirculation of carbonmonoxide are low.

In operation of the new process, the charge mixture is conveniently abriquetted mixture of magnesia and petroleum coke, in stoichiometricproportions. However, other magnesia-containing materials, such ascalcined dolomite, and other forms of carbonaceous reducing agent may beused.

The quenching liquid used is preferably a substantially non-volatilemolten metal absorbent miscible with magnesium, such as lead or aleadmagnesium alloy. However, other quenching liquids, includingvolatile metals miscible with magnesium, molten salt mixtures, and heavyoils, may be employed. Likewise, the quench step is not limited to theuse of liquid media, since other shock-chilling means, such as a rotarydrum condenser, are contemplated as within the invention.

It is to be understood that the foregoing description is illustrativerather than strictly limitative, and that the invention is co-extensivein scope with the following claims.

This application is a continuation-in-part of application Serial No.423,944, filed December 22, 1941.

Attention is directed to a co-pending application. Serial No. 584,630,filed March 24, 1945 by T. H. McConica, III, et al., in which claims areasserted to the constructional details of the quench condenser,disclosed, but not claimed, in this application.

The invention claimed is:

1. In a method of producing magnesium, the steps which comprise heatinga mixture of a magnesium oxide source material and a carbonaceousreducing agent at a pressure of at least about 0.5 atmosphere and at atemperature suflicient to form a vapor mixture of magnesium and carbonmonoxide, expanding the vapor mixture from such pressure to a. reducedpressure between 0.1 and about 2.5 inches of mercury absolute, andrapidly cooling the vapor mixture at the latter pressure to atemperature below about 650 C.

2. A process according to claim 1 in which the vapor mixture is expandedto a pressure between about 0.2 and about 1.0 inch of mercury absolute.

3. In a method of producing magnesium by the thermal reduction ofmagnesium oxide with carbon, the steps which comprise heating a mixtureof magnesium oxide and carbon at approximately atmospheric pressure andat a temperature sufiicient to form a, vapor mixture of carbon monoxideand magnesium, expanding the vapor mixture from such pressure to areduced pressure between 0.1 and about 2.5 inches of mercury absolute,and quenching the vapor mixture at the latter pressure to a temperaturebelow about 650 C.

4. In a method of producing magnesium, the steps which comprise heatinga charge mixture of magnesium oxide and carbon at a pressure of at leastabout 0.5 atmosphere and at a temperature suillcient to form a vapormixture of carbon monoxide and magnesium, expanding the vapor mixturefrom such pressure to a reduced pressure between 0.1 and about 2.5inches of mercury absolute, cooling the vapor mixture while at saidpressure to a temperature which is not below its upper reversiontemperature, and then quenching the cooled vapor mixture while still atthe said reduced pressureto a temperature below about 650 C.

5. In a process 01 producing magnesium, the steps which comprise:heating a charge consisting essentially oi magnesium oxide and carbon ina confined zone at a pressure of at least 0.5 atmosphere to atemperature sufilcient to convert the charge to a vapor mixture ofmagnesium and carbon monoxide, expanding the mixture directly from theconfined zone through a restricted orifice into a vapor conduitmaintained at a reduced pressure between 0.1 and about 2.5 inches ofmercury absolute, conveying the vapor mixture through said conduit to aquench zone maintained at substantially the same reduced pressure, andquenching the vapor mixture in the latter zone to a temperature belowabout 650 C.

6. A process according to claim 5 in which the charge mixture is heatedat approximately atmospheric pressure and in which the vapor mixture isexpanded to a pressure between about 0.2 and about 1.0 inch of mercuryabsolute.

'7. A process according to claim 5 in which the pressure in the heatingzone is maintained by admitting carbon monoxide thereto and controllingthe rate of such admission in response to variations in the pressure inthe heating zone.

8. A process according to claim 5 in which the pressure in the heatingzone is maintained by regulating the rate at which the charge is fed tothe heating zone so that the sum of the partial pressures of theevolving magnesium and carbon monoxide is substantially equal to thetotal pressure in the heating zone.

9. A process for producing magnesium which comprises heating a mixtureof a magnesium oxide source material and a carbonaceous reducing agentin a confined zone at a pressure of at least about 0.5 atmosphere to atemperature sufilcient to convert the charge to a vapor mixtureof-magnesium and carbon monoxide, expanding the mixture from theconfined zone through a restricted orifice into a quench zone maintainedat a reduced pressure between 0.1 and about 2.5 inches of mercuryabsolute and therein passing the expanded mixture into intimate contactwith a substantially non-volatile molten metal absorbent miscible withmagnesium and supplied at a temperature below about 650 C. to condensethe magnesium vapor in the absorbent, and removing absorbent from thequench zone and recovering magnesium therefrom.

10. A process for producing magnesium which v at at least 0.5atmosphere; expanding the evolvin: vapor mixture through the restrictedoutlet into a quench zone at a pressure between about 0.1 and about 1.0inch of mercury absolute and therein passing the expanded vapor mixtureinto intimate contact with a substantially non-volatile molten metalabsorbent consistingpredominantly of lead and maintained at atemperature below about 650 C. to condense the magnesium vapor in theabsorbent: exhaustinz uneondeneed carbon monoxide from the quench zoneat a rate euiiicient. to maintain the pressure in the zone within theaforesaid limits: and withdrawing the absorbent from the zone andrecovering meaneaium therefrom.

THOMAS H. McCONICA. III.

